The present invention relates to a bit adaptor comprising a guide sleeve in which a drill bit is received and having a receiving member, especially a square or hexagonal connector, for a screwdriver bit, whereby the blind bore for receiving the drill bit has follower elements at its bottom for a torsional connection of the drill bit inorder to entrain and rotate the drill bit.
Such a bit adaptor is known from German Offenlegungsschrift 39 15 458. This drill bit adaptor is characterized by an especially good transmission of torsional forces even when sensitive, i.e., brittle, hard metal plates project laterally and toward the center. The torsional transmission in this known device is performed by lateral flanks of the drill bit head which contact lateral walls of the correspondingly shaped blind bore or of a guide sleeve.
Even though the known device is especially suitable for transmission of torque, the drill bit has the tendency to jam within the bit adaptor when high axial forces are exerted. On the other hand, the operator has the tendency, based on his practical experience, to exert a very strong axial load onto the bit adaptor especially for screwing in Phillips-head screws. This is so because, on the one hand, when an accidental slipping of the screwdriver bit on the Phillips-head screw occurs, at least one Phillips-head screw will become defective and must be removed in a cumbersome manner and, on the other hand, a comparatively great wear on the edges of the screwdriver bit will be generated which, over time, will cause the screwdriver bit to slip even more often.
The bit adaptor can be released from the drill tool by pounding on it with a hammer or a similar device when it is jammed. However, the operator will usually try to remove the bit adaptor from the drilling tool simply by pulling. This, however, constitutes a greater risk for injury because the operator can easily be hurt by the sharp and practically helically extending cutting edges of the drill bit. This risk is even greater for inexperienced hobby craftsmen which use such drill bits only infrequently.
Another bit adaptor is known from German Offenlegungsschrift 25 09 961. This bit adaptor in one embodiment has at its rearward end a rotary follower so that the drill bit must be practically threaded into the bit adaptor in order to be useable. This embodiment is impractical because the insertion and removal always requires multiple turns and actuation of the drilling tool until the bit adaptor has been completely inserted. Furthermore, an actuation for turning the screws to the left, i.e., for loosening screws is problematic because the drill bit has the tendency to become detached from the bit adaptor.
In another embodiment, a longitudinal slot is provided which ends bluntly and essentially provides a following action by the hard metal plates of the drill bit projecting from the drill bit. In this embodiment, the hard metal plate is greatly loaded with torsional forces whereby the drill bit is, in general, not suited for such loading. In some cases the radial projection of the hard metal plate is only so minimal that the hard metal plate can be engaged by the longitudinal slot only to a minimal extent and, due to thus resulting minimal contact surfaces, is extremely loaded at its outer edges. Since the hard metal plates are very hard but also brittle, they have the tendency to break when unfavorable load conditions occur.
Thus, this solution is suitable only when drill bits with identical dimensions, especially with identical thread diameter are used. Even though the actual diameter of the drill bits of different manufacturers is of the same size, the diameter of the helical thread, i.e., the projecting portions of the hardmetal plates, will fluctuate from manufacturer to manufacturer. For a reduced actual dimension the known drill bit cannot be exactly guided while for an excessive actual size the thread of the drill bit has the tendency to become jammed within the guide bushing?
Also, both variants of the bit adaptor known from German Offenlegungsschrift 25 09 961 experience great wear so that it is not surprising that this solution, despite its advantages, has not been accepted by the customer.
It is therefore an object of the present invention to provide a bit adaptor of the aforementioned kind which, with respect to the employed materials on the one hand and the observance of tolerances for the dimensions of the drill bit on the other hand, is less critical and is also, in general, suitable for insertion of bolts.